BEIJING -?Authorities have arrested 989 people over the last six months as part of a national crackdown on the manufacture and sale of clenbuterol, an illegal fat-burning drug that is sometimes used as an additive in pig feed, a public security official said Monday.

Police busted a criminal ring that was involved in the manufacture and sale of clenbuterol in 63 cities nationwide, according to Xu Hu, a senior official from the Ministry of Public Security.

Police seized 2.5 metric tons of clenbuterol and closed six illegal laboratories, 12 production lines, 19 processing and storage sites and 32 "underground" factories during the crackdown, Xu said.

Authorities launched a food safety campaign in March of this year, with a focus on clenbuterol. A total of 2,000 police officers in the provinces of Hunan, Sichuan, Hubei and Henan, as well as Tianjin Municipality, participated in the latest bust.

Farmers sometimes use clenbuterol, a toxic chemical that is banned as a food additive in China, to make their pigs more lean.